Antisinging gain controlled repeater



E-W A MPLIFIER June 7, .1949. R. H. HERRICK 2,472,211

ANTISINGING GAIN CONTROLLED REPEATER Filed Dec. 13, 1945 n: E E g n: o m

DISORIMINATOR I v INVENTOR.

ROSWELL H. HERRICK AITORNEY Patented June 7, 1949 UNITED STATES PATENT OFFICE ANTISINGING GAIN CONTROLLED REPEATER Roswell H. Herrick, Chicago, Ill'., assignor to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application December 13, 1945', Serial No. 634,769

12 Claims. 1

The present invention relates generally totwoway transmission systems which employ separate amplifying meansfor transmitting signals in each direction and signal controlled means for governing the gain of the amplifying means and, more particularly, toimprovementsin the method of determining the signal direction and controlling the gain of the amplifying means accordingly.

In the usual two-way signal transmission systemv hybrid transformers and balancing networks are used to couple the amplifiers to the transmission line so as to prevent the output of one amplifier from afifecting the input of the other amplifier. Such an: arrangement will operate satisfactorily only if the impedanceof the lines remains constant. However,-. this condition is seldom realized in practice when the line may be exposed to varying weather conditions which materially change the characteristics of the line. When the line characteristics change the hybrid network becomes unbalanced and the input and output circuitsf the two'ampli-fiers are nolong'er completely isolated; In order to" prevent singing, the gain of the amplifiers must therefore be limitedtoa value such that the transmission gain is less than the transmission loss; in theloop circuit. formed by the two amplifiers and hybrid transformers, under the worst conditions of uhbalance' likely to occur;

In order to increasethe usable gain" of the amplifiers various methods have been proposed for automatically adjusting the gain in response to the arrival' of a signalat the input of'either of the amplifiersso asto-increase the gain of the active amplifier and reduce thegain of the inactive amplifi'er. A novel' approach to this problem is disclosed in my copending application, Serial No. 533,205; filed April 28, 1944;, now Patent No. 2,46855'2, issued April 26, 1949, wherein it is pro+ posed to insert a Tpad in' each line and operate the gain control circuits. in accordance with the sensed the difference in signal'voltages appearing across the'line on eitherside of the associated T pad. The same control principle is employed-in the present case wherein the inherent loss in a hybrid transformer is'employed to determine the signaldirection in place of T'pads; The improved stability of the former system is retained while increased efiicienc'y is secured through-the elimination of the loss introduced'by theinsertionof the'l, pads in each line.

The principal object of the present invention is to provide an improved control circuit for governing the gains of the. amplifiers in a two-way voice repeater in accordance with the direction of signal transmission. v

A feature of the above mentioned control circuit is the provision of a novel circuit arrangement for d'eionizing. a gaseous discharge gain com trol tube in a voicerep'eate'r in response to the cessation of signal tranmiss ion.

Another feature of the control circuit is the provision of means for automatically regulating the gain ofthe control amplifiers in a voice-controlled swit'ching system in accordance with the signal strength to prevent overloading thereof and consequent false operations.-

Other objects and features of the invention will appear upon a further perusal" of the specification taken in conjunction with the accompanying drawing which illustrates the invention in the formof a schematic diagram.

In the illustrated embodiment of the invention two oppositely directed amplifiers are coupled to the east and West section's-of a transmission line by the usual hybrid transformers and balancing networks. A signal direction discriminating circuit isassociated with eachhybrid transformer. Each of these circuits-is arranged to develop two direct current control potentials of polarities dependent uponthedirection of signal transmission through the associated hybrid transformer. The direct current control potentials developed by each discriminator are combined and used to fire either of two gaseous di's'cha'r'getub'es dependcut on the signal direction; E'ach gaseous discharge tube is arranged to enable one amplifier and disable the other. A vacuum tube and relay circuit is associated with each gaseous discharge tube'to deionize it upon cessation of signal transmission. Duplicate discriminators are employed to provide equal sensitivity for either direction of signal transmission. I

Referring nowmore particularly to the drawing; there is shown an'E-Wamphfier and a W-E amplifier'coupled to the lines E and Why hybrid transformers I 0 and l l in the usual manner. The discriminators I and 2 are associated with the hybrid transformers l0 and H, respectively, to determine the'direction of signal transmission therethrough. The gaseous dischargegain controltubes andass'ociatedvacuum tubes and relays are shown in the center of the drawing. Each of the hybrid transformers I0 and Il includes a pair of center-tapped secondary, or line, windings and a primary, or plate, winding. Discriminator I includes twovacuum tubes l2 and to the primary windingrof hybridtransformer If) and the second of which has its grid circuit coupled to line W through transformer I4. The primary to secondary turns ratio of transformer i4 is the same as the total secondary to primary turns ratio of hybrid transformer l0.

Now when a signal is impressed on the primary winding of transformer Hl the voltage induced in the secondary windings is divided equally between the balancing network l5 and. the line W. The voltage induced in the secondary Winding of transformer l4 will therefore'be equal'to onehalf the voltage across the primary of transform-- er ID. If a signal is receivedover line W the voltage which appears across the primary of transformer l4 will divide between the balancing network l5 and the secondary windings of transformer H! in direct proportion'to their'impedances. Assuming that the impedance of the output circuit of the E-W amplifier as viewed from the secondary of transformer Ill is equal to the impedance of the balahcingrietworkf [5, which is in turn equal to the impedance of lineW. then the line voltage .will divi de':equal ly between the balancing networklfi and the secondary windings of transformer Ill. is the usual condition since the optimumfloadfora" power amplifier tube is approximatel'y'twice the plate resistance of the tube. The voltage induced in the secondary of transformer l4 will therefore be twice that induced in the primary of transformer i 6. Thus it may be seen that ;for"E- W signal transmission the 'voltageijimpressed. on the grid of tube l2 will be twiceithatirnpresseri on the grid of tube I73, and for W-E signal transmission the voltage impressedon the grid of tube [3 will be twice thatimpressed on'the grid of tube l2. A slightly higher ratio is actually realized since the hybrid transformer lfl isnot 100% 'efiicient. Vacuum tubes 12' and]! 3; amplify'the voltages impressed on their grids 'anddevelop corresponding voltages in the secondary windings of the transformers connected initheir plate circuits. The voltages developed inftl'iese secondary windings are rectified and differentially combined in each of two control circuits. Thus'winding I6 is connected across resistor 24' through rectifier which is poled so as 'to'nia'ke the upper end of resistor 24 positive with respect to the lower end thereof, and winding 'l8'is connected across resistor 25 throughrectifier'Zl, which is poled so as to make the upper end of resistor 25"negative with respect to the lower end thereof. The voltage between the upper endof resistor 24 and ground will therefore be positive or negative depending on which tube,"l2.lor 'l3; receives the greater signal voltage. Windings I1 and I9 are similarly connectedacross resistors 26 and 21 through rectifiers 22 and 23, respectively; so that the voltage between the lowerend of resistor 21 and ground is equal in magnitude but opposite in polarity to that between the. upper end of resistor 24 and ground. Small condensers 28 and 29 are provided for filtering these voltages.

Discriminator 2 is identical to that just described and operates in the same manner to produce voltages of the polarities indicated across resistors 30 to 33, which voltages are filtered by small condensers 34 and 35. For east to west signal transmission the voltages across resistors 24, 26, 30 and 32 predominate over the voltages across resistors 25, 21, 3| and 33 thereby making points A and B positive and'negative, respectively, with respect to ground. Similarly, for west to east signal transmission points A and B become negative and positive; respectively.

When point A is made positive in response to speech signals received over line E, gas discharge tube 36 fires and the associated vacuum tube 31 is rendered conductive. Prior to the time that tube 36 fired, the current flowing through resistor 38 from the plate supply lead (+B) in series with resistor 39 is such that the voltage impressed on the screen grid 'of tube 40 causes the E-W amplifier to have a desired standby gain. Similarly the voltage impressed on the screen grid of tube 4! from resistor 42 causes the W-E amplifier to'have a desired standby gain. Normal voltages are impressed on the cathodes of tubes 40 and M from resistors 42 and 38, respectively, at this time. When tube 36 fires it increases the current flow through resistor 38 thereby raising the screen grid voltage of tube 40- and the cathode voltage of tube 4| to increase the gain of the E-W amplifier by a predetermined amount and to decrease the gain of the W-E amplifier by a corresponding amount. As relay 43, which is connected in the anode circuit of gas discharge tube 36, is rendered slow-to-operate by means of a copper slug; relay 44, which is connected in the anode circuit of tube 31, will operate first in response to the positive control voltage on point A. When relay 44 operates it connects condenser 45 across its winding at contacts 46 to render the relay slow-to-release, and closes an alternative circuit to relay 43 at contacts 41. Relay 43 subsequently operates and opens a further point in the initial anode circuit for tube 36 at contacts 48. 'When speech transmission ceases the positive potential on the grids of tubes 36 and 31 rapidly falls to zero, thereby rendering tube 31 nonconductive. The condenser 45 connected in multiple with relay 44 renders it slowto-release in order to provide a short hangover interval, whereby the amplifier gains will not be afiected by the brief interruptions normally occurring in conversation. If speech transmission is interrupted for a longer interval than the hangover interval, relay 44 will release and interrupt the circuit to relay 43 in series with tube 36 at contacts 41. Tube 36 is instantly deionized thereby stopping the increased current flow through resistor 38 to restore the amplifier gains to normal by lowering the screen voltage of tube 40 and the cathode voltage of tube 4|. Relay 43 restores and recloses the anode circuit of tube 36 at contacts 48. Tube 36 does not fire again at this time as the potential impressed on its control electrode is below the critical value necessary to initiate a discharge therein.

When speech signals are received over line W, point B becomes positive and point A becomes negative. The positive potential of point B causes tube 49 to become conductive and causes a discharge to be initiated in gas discharge tube 50. The increased current flow through resistor 42, caused by the ionization of tube 50, raises the screen grid voltage of tube 4| and the cathode voltage of tube 40. The ain of the W-E amplifier is thereby increased a predetermined amount while the gain of the E-W amplifier is reduced by a corresponding amount. Relay 51 operates, connects condenser 52 in multiple with its winding at contacts 53 to render the relay slow to release, and closes an alternate circuit to tube at contacts 54. Relay 55 operates after a brief delay and opens a further. point in the initial anode circuit for tube 50 atcontacts 56. When the signal transmission ceases the potential of point Brapidly decays tozero. Tube 49 becomes non-conductive causing relay 5| to restore after 5 the hangover intervaland opens. the anodecirc'uit .of tube 50 at contacts, 54. Tube 50..is. .thus deionized causing the gains of the. amplifiersto be returned to normal. When relay 55subsequently restores it recloses the anodev circuit for tube 50 at contacts 56.

The voice-operated gain. control. is arranged to permit break-in operation.. Thus if speech transmission is taking place in the east-to-west direction, tube 36 will be ionized to enable the E-W amplifier and disable the W-E amplifier. If speech signals are received over line W at this time, they will tend to reverse the polarity of points A and B. The party who was'speaking first has an advantage because the -sp eech signals received over line E are amplified to a high degree in the E-W amplifier before they affect discriminator I. On the other hand, the speech signals received over line W are amplified to a lower degree in the W-E amplifier before they affect discriminator 2. Speech signals from either direction acting directly on the discriminators without amplification have equal effects. Thus it is necessary that the signals received over line W be stronger than those received over line E in,

order to reverse the polarity of points A ad B to thereby initiate a discharge in tube 50. When tube 50 fires it raises the screen grid voltage of tube tl and the cathode voltage of tube 40, thereby disabling the E-W amplifier. When tube 36 is deionized it lowers the screen grid voltage of tube 40 and the cathode voltage of tube 4|, thereby further disabling the E-W amplifier and enabling the W-E amplifier.

Because the voltages impressed on the input of each discriminator are much greater for one direction of speech transmission than the other due to amplification in the E- W, or W-E,, amplifiers, it is desirable that some provision be made to avoid overloading the discriminators. The discriminators therefore include. an automatic gain control circuit. In the absence of such a circuit the value of the signal voltages impressed on the discriminator would haveto be limited to a value below the overload point forthe direc-.

tomatic gain control circuit enables maximum sensitivity to be attained for either directionof signal transmission. The potential developed across the primary of transformer l0, during,signal transmission in either direction, is impressed on resistors 57 and 58 in series throughcondenser 59. The voltage appearing across resistor 51-, is impressed on resistor 60 through rectifier 6|. A unidirectional potential proportional to the strength of the signals impressed on the input of discriminator l is thereby developed across resistor Bil. Condenser 62 is shunted across resistor B0 to prevent speech components from appearing thereacross. The voltage across resistor 60 is impressed on the grids of tubes l2 and [3 to vary the gain thereof inversely with the strength of the speech signals impressed thereon. A similar automatic gain control circuit is provided for discriminator 2.

While in the system as described the gain of one amplifier is decreased when the gain of the other is increased, it should be understood that the standby gain may be made sufliciently low so that when the gain of either amplifier is increased the net 1oop gain will be less than the net loop loss without decreasing the gain of the other vtwo oppositely directed voice repeaters connected, between two lines by hybrid transformers,

control means operative to increase .thegain-of eitherrepeater, a signal direction discriminator differentially coupled to one of the lines and to the primary winding. of the corresponding hybrid transformer, said discriminator operated in response to signal transmission in either direction to operate said control means so as to increase the gain of the active repeater.

2. In a two-way transmission system including two oppositely directed voice repeaters connected between two lines by hybrid transformers, control means operative to increase the gain of either repeater, two signal direction discriminators each differentially coupled to one of the lines. and to the primary winding of the correspondinghybrid transformer, said discriminators concurrently operated inresponse to signaltransmission in either direction to operate said control means so .asto increase the gain of the active repeater.

3, In a two-way transmission system including two oppositely directed voice repeaters connected between two lines by hybrid transformers, control means operative to concurrently vary. the gains of the repeaters in opposite senses, a signal direction discriminator differentially coupled to one of the lines and to the primary winding of. the corresponding hybrid transformer, said discriminator operated in response to signal transmission in either direction to operate said control means so asto increase the gain. of the active repeater and decrease the gain of the inactive repeater.

4. .In a transmission system including a bilateralsignaltransmission channel and two oppositely directed unilateral signal transmission channels coupled thereto by a hybrid transformer, a signal direction discriminator differentially coupled to the bilateral channel and to the primary winding of the hybrid transformer, means included in each of the unilateral channels for varying the transmission efficiency thereof, said discriminator operated in response to signal transmission in either direction to cause said means to increase the transmission efficiency of the active unilateral channel.

5. In a signal transmission system including a bilateral signal transmission channel, a balancing network, a three winding transformer, two voice repeaters, two of the windings of said transformer being serially connected between said bilateral channel and said balancing network, the input circuit of one of said repeaters being connected between taps on said two windings, and the output of the other repeater being connected to the third winding of said transformer, means for deriving one voltage from the bilateral channel in response to signal waves, means for deriving a Second voltage from the third winding of the transformer in response to signal waves, the ratio between said two voltages being greater or less than'one'depending on the signal direction, and -mean's for increasing the gain of either one of 'the repeaters in accordance with whether said i-ratio is greater or less than one.

' 6. In a two-way transmission system terminating two lines and including two oppositely directed voice repeaters, means for deriving one signal voltage in response to incoming signal waves transmitted over one of said lines in one direction and for deriving a second signal voltage in response to signal waves received from the output of the other of said lines during signal transmission in the opposite direction, the ratio of said first signal voltage to said second signal voltage being greater than one for signal transmission in said one direction and the ratio of said first signal voltage to said second signal voltage being less than one for signal transmission in said opposite direction, a control circuit for each repeater, rectifying means for producing a unidirectional potential in each of said circuits from each of the derived signal voltages, said rectifying means being poled so that the net unidirectional potential in each control circuit is positive or negative dependent on the said ratio and direction of signal transmission, and means governed in accordance with the polarity of the net unidirectional potentials in each of said control circuits for regulating the gain or" the corresponding repeater.

7. In a transmission system including a bilateral signal transmission channel and two oppositely directed unilateral signal transmission channels coupled thereto in conjugate relationship, means for deriving one speech signal voltage from the bilateral channel and another speech signal voltage from one of said unilateral channels, said derived speech signal voltages having a ratio greater or less than one depending on the direction of signal transmission, a voice repeater in each unilateral channel, a control circuit for each repeater, rectifying means for producin a unidirectional potential in each of said circuits from each of the derived signal voltages, said rectifying means being poled so that the net unidirectional potential in each control circuit is positive or negative dependent on said ratio and the direction of signal transmission, and means governed in accordance with the polarity of the net unidirectional potentials in each of said control circuits for regulating the gain of the corresponding repeater.

8. In a transmission system including a bilateral signal transmission channel and two oppositely directed voice repeaters coupled thereto by a hybrid transformer, a signal direction discriminator differentially coupled to the bilateral channel and to the primary winding of the hybrid .transformer,"andrmeans controlled by said discriminator in response'to signal transmission in either directionrior governing the gains of the repeaters accordingly.

9..A transmission system as claimed in claim 8 wherein said signal direction discriminator includes amplifying-means, and means operative to regulate the gain of the amplifying means in accordance with the strength of the speech signals impressed thereon.

10.,In atransmission system including two oppositely-directed voice repeaters, a voice-operated control circuit-having a signal direction discriminator and including amplifying means in said discriminatorfor governing the gains of said repeaters in accordance with the direction of signal transmission, and means for varying the gain of said amplifying means in said discriminator inversely with the voice signal strength over said control circuit.

11. A two-way transmission system as claimed in claim 6 in which said last means includes a gaseous discharge tube operated by a positive net unidirectional potential, said gaseous discharge tube havingananode circuit including a resistor through which current flows in response to the operation thereof, and means responsive to the flow of currentthrough said resistor for increasing the gain of the corresponding repeater.

12. Atransmission system as claimed in claim 4 in which said means includes a gaseous discharge tube operated in response to the operation of said signal direction discriminator, a vacuum tube also operated-in response to the operation of saidsignal direction discriminator, and means including said vacuum tube for maintaining said gaseous discharge tube operated a certain interval independent of the operation of said discriminator to thereby maintain the increased efilciency of the active unilateral chan nel for a certain interval after the cessation of signal transmission.

ROSWELL H. HERRICK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,866,592 B. Jonnson July 12, 1932 2,017,645 Barney Oct. 15, 1935 ,106,337 Black Jan. 25, 1938 2,208,160 H. McCreary July 16, 1940 2,270,789 Smith Jan. 20, 1942 2,371,291 Herrick Mar. 13, 1945 2,396,175 Herrick Mar. 5, 1946 

